Painting new plaster: mist coat, primer and how many coats

Bare plaster soaks up paint, so the first coat has to soak in and seal it: either a mist coat, which is emulsion thinned with water as the tin says, or a primer-sealer made for bare plaster. The top coats, usually two, go on after that, once the plaster is completely dry; the paint’s instructions decide the dilution, the drying time and whether two top coats are enough.

Why bare plaster is different

• It is porous. A full-strength coat is drawn into it unevenly and can dry before it grips, and is then more likely to peel.
• The tin’s coverage is usually for a sealed surface. On bare plaster the first coat goes nowhere near as far.
• Filled patches behave the same way on a painted wall: they soak up more than the paint around them and can show through.

Mist coat or primer-sealer

	Mist coat	Primer-sealer
What it is	Emulsion thinned with water	A product made to seal bare plaster
Which product	One whose tin says it suits a mist coat; some say they do not	One whose tin says it is for bare plaster
How to use it	Thinned as the tin says	As the tin says
Then	Top coats of emulsion	Top coats of emulsion

Neither is right for every paint. The paint’s instructions for bare plaster decide, and that is what to buy, not a guess.

Wait until it is dry

New plaster must be completely dry before any paint goes on. How long that takes depends on the plaster, its thickness, the room and the weather, so the plaster’s and the paint’s instructions set the time, not a rule of thumb.

The usual sign is the colour. Wet plaster is darker and lightens evenly as it dries, so darker patches mean it is not ready. If patches stay dark long after the rest has dried, find out why before you paint; see the damp section of preparing walls before painting.

How many coats

Usually one mist coat or primer-sealer and then two top coats. The paint’s instructions decide whether two top coats are enough after it, and a strong colour or a patchy result can need a third. Two top coats straight onto bare plaster may not cover.

Worked example

The example room’s walls, 34.484 m² of new plaster, at the example figures of 10 m² per litre per coat and 10% extra:

• Two top coats: 34.484 × 2 ÷ 10 × 1.1 = 7.586 L
• A mist coat, counted as one more coat’s worth: 34.484 × 1 ÷ 10 × 1.1 = 3.793 L
• All three, if the mist coat is the same emulsion: 34.484 × 3 ÷ 10 × 1.1 = 11.38 L

In 2.5 L tins that is 5 tins instead of 4. If the ceiling is new plaster too, its 15.12 m² adds another 1.663 L for its mist coat.

Treat the extra coat as a planning figure, not a measurement. Part of a mist coat is water, and bare plaster drinks more than a sealed wall, so the real figure depends on the dilution and the plaster. A primer-sealer is a separate product with its own line: 34.484 m² ÷ the coverage on the primer’s tin, rounded up to its tin sizes.

Walls stripped back to plaster

A wall stripped back to the plaster counts as bare plaster too. Older paint can contain lead, and textured coatings applied before 2000 can contain asbestos: do not sand or scrape a coating you are not sure about; get advice first.

Do it with BuildBatch

Open the Room Refresh Batch with paint chosen and answer “New or bare plaster” to the question about the walls. The batch adds “Mist coat or primer-sealer for bare plaster” as a Check first line with the area beside it, and warns that two top coats straight onto bare plaster may not cover. To count a mist coat of the same emulsion into the wall paint, raise the coats by one.

What this guide does not cover

Plastering or skimming, and repairing plaster that is cracked or coming away. Diagnosing damp. Exterior render and masonry. Woodwork. Which paint to choose: the tin, and the maker’s data sheet, decide what suits bare plaster.

Related guides

• Preparing walls before painting
• Paint coverage explained: m² per litre, per coat
• What you need to paint a room: the complete checklist